In the Aug. 1 episode, Maher showed the cover of the book, with 13-year-old Brenna Strong standing in front of her parents, both of whom have pistols holstered to their hips.

Maher asks, "If mom and dad are both safe because they're packing, why are they using their daughter as a human shield?"

The book is written by Brian Jeffs and Nathan Nephew, founders of Michigan Open Carry.

On the book's website, the authors say: "If you open carry and have a difficult time explaining why to your family and friends, or if you want to learn about the open carry of a handgun, or if you've wondered if open carry is right for you, then this book is what you need."

Jeffs and Nephew continue: "Before writing this, we looked for pro-gun children's books and couldn't find any. Our goal was to provide a wholesome family book that reflects the views of the majority of the American people, i.e., that self-defense is a basic natural right and that firearms provide the most efficient means for that defense."

Jeffs told CNN he has gotten death threats from people opposed to his position, but said he and Nephew are working on a sequel.

A children's book promoting the right to bear arms has seen a sharp increase in sales since being mocked by comedian Bill Maher, its publisher says. "My Parents Open Carry" was originally published in 2012 and had few sales, but has quickly risen to the top of...